1. Field of the Invention
This invention relates generally to armor systems for structural protection against ballistic impact or explosive blast, and more particularly to the use of a metallic foam as the shock energy-absorbing element in a multi-layer armor system.
2. Description of the Related Art
With increasing terroristic violence and military action, there is a need for improved structural protection against ballistic impact from projectiles or blast protection from explosives. Such structural protection can be built into the infrastructure of a building to reinforce the building, or certain rooms within a building, against attack. Structural protection is also useful in vehicles, illustratively military vehicles, such as tanks, or civilian VIP vehicles. Presently, a multi-layer armor system is employed in known vehicular applications.
A typical configuration for the armor system in medium weight military vehicles, for example, consists of a high strength strike face (either a metal or a ceramic plate), bonded to a ceramic tile, which is subsequently bonded to a metallic backing plate. In this configuration, the ceramic tile breaks-up or deforms an incoming projectile, and the metallic backing xe2x80x9ccatchesxe2x80x9d the extant penetrator and ceramic fragments. The high strength strike plate aids the ceramic tile by providing front face confinement, and may, in some cases, protect the ceramic tile from field damage.
Upon projectile impact at typical ordnance velocities, a stress wave is generated and propagates through the ceramic tile. Reflections from boundaries and subsequent stress wave interactions result in tensile stress states and attendant microcracking. Microcracking due to these impact-induced stress waves weakens the ceramic tile, allowing a projectile to penetrate more easily. In armor system designs utilizing a metal strike plate over ceramic tile, stress waves from a projectile impact on the metal strike plate can run ahead into the ceramic, and failure may initiate prior to contact of the projectile with the ceramic tile. There is, therefore, a need for an armor system having an improved shock-absorbing element, and more particularly, a shock-absorbing element that gives more control of behind-the-target effects, such as backface deformation and spalling.
Metallic foams with a high fraction of porosity are a new class of materials which have attributes that lend themselves to various engineering applications, including sound and heat isolation, lightweight construction, and energy absorption. The latter two applications, in particular, make use of the unique characteristics of a metallic cellular material, specifically the combination of its comparatively high specific strength and its characteristic non-linear deformation behavior. As will be described more completely hereinbelow, certain metal foams are effective in containing rearward deformation of a target under high-speed impact, and therefore are useful in controlling backface deformation and spalling. Moreover, metal foams are capable of mitigating impact-induced stress waves thereby delaying damage to ceramic layers in armor systems employing same.
It is, therefore, an object of the invention to provide an armor system incorporating metal foam as a shock energy-absorbing element to improve protection of equipment and personnel behind the target.
It is a further object of the invention to provide an armor system incorporating metal foam as a shock energy-absorbing element to control behind-the-target effects as a result of backface deformation caused by the high energy impact of a projectile.
The foregoing and other objects, features and advantages are achieved by this invention which provides a metallic foam as a shock-absorbing element in a multi-layer armor system. In preferred embodiments, the metallic foam has a closed-cell pore structure and a high fraction of porosity, preferably ranging from about 50-98 percent by volume.
Metallic foams useful in the practice of the present invention may be, but are not limited to, metal foams of aluminum, steel, lead, zinc, titanium, nickel and alloys or metal matrix composites thereof. Metal foams can be fabricated by various processes that are known for the manufacture of metal foams, including casting, powder metallurgy, metallic deposition, and sputter deposition. Exemplary processes for making metal foams are set forth in U.S. Pat. Nos. 5,151,246; 4,973,358; and 5,181,549, the text of which is incorporated herein by reference.
U.S. Pat. No. 5,151,246, for example, describes a powder metallurgy process for making foamable materials using metallic powders and small amounts of propellants. The process starts by mixing commercially available metal powder(s) with a small amount of foaming agent. After the foaming agent is uniformly distributed within the matrix material, the mixture is compacted to yield a dense, semi-finished product without any residual open porosity. Further shaping of the foamable material can be achieved through subsequent metalworking processes such as rolling, swaging or extrusion.
Following the metalworking steps, the foamable material is heated to temperatures near the melting point of the matrix metal(s). During heating, the foaming agent decomposes, and the released gas forces the densified material to expand into a highly porous structure. The density of the metal foams can be controlled by adjusting the content of the foaming agent and several other foaming parameters, such as temperature and heating rate. The density of aluminum foams, for example, typically ranges from about 0.5 to 1 g/cm3.
Strength, and other properties of foamed metals can be tailored by adjusting the specific weight (or porosity), alloy composition, heat treatment history, and pore morphology as is known to those of skill in the art. In advantageous embodiments, the metallic foam will have high mechanical strength.
Metal foams are easily processed into any desired shape or configuration by conventional techniques, such as sawing drilling, milling, and the like. Moreover, metal foams can be joined by known techniques, such as adhesive bonding, soldering, and welding.
In certain preferred embodiments of the invention, the shock-absorbing element is closed-cell aluminum foam, and in a specific illustrative embodiment, the shock-absorbing element is closed-cell aluminum foam with a porosity of 80 percent by volume.
In device embodiments of the present invention, a multi-layered armor system, suitable for structural protection against ballistic impact or explosive blast, such as armor systems used in connection with military armored vehicles, includes one or more layers of a metal foam as a shock energy-absorbing element.
As used herein, the term xe2x80x9cmulti-layer armor systemxe2x80x9d means at least two plates of metal, metal foam, ceramic, plastic, and the like, known or developed, for defense or protection systems. In the present invention, the multi-layer armor system includes at least a strike plate, or buffer plate, bonded or otherwise held in communication with, a shock-absorbing element that is a layer of metallic foam.
As described hereinabove, the metallic foam preferably has a closed-cell pore structure and a high fraction of porosity. Illustratively, the metallic foam may be aluminum, steel, lead, zinc, titanium, nickel and alloys or metal matrix composites thereof, with porosity ranging from about 50-98 percent by volume. In a particularly preferred embodiment of the invention, the metallic foam is a closed-cell aluminum foam having a porosity of 80 percent by volume.
The term xe2x80x9cstrike platexe2x80x9d refers to a high strength metal or ceramic plate that has a front face surface that would receive the initial impact of a projectile or blast. The back surface of the strike plate is adjacent to a first surface of the shock-absorbing element that, in the present invention, is a sheet or layer of metallic foam. It is to be understood that the term xe2x80x9cstrike plate,xe2x80x9d as used herein, refers to any buffer plate of a high strength material that receives impact or impact-induced stress waves prior to a shock-absorbing element.
The strike plate may be a flat sheet of a high strength metal, ceramic or polymer-based composite, such as a fiber-reinforced polymer composite.
In a preferred embodiment, the multi-layer armor system of the present invention further includes a deformable backing plate bonded to, or otherwise held in communication with, a face surface of the metallic foam sheet or layer opposite, or distal, to the surface contiguous to the strike plate. The backing plate illustratively is a sheet of a deformable metal, such as titanium, aluminum, or steel.
In a specific illustrative embodiment of a multi-layer armor system in accordance with the invention, a shock-absorbing layer of metallic foam is sandwiched between a high strength strike plate and a deformable backing plate. Of course, the multi-layered armor system may comprise additional elements, in any sequence, and the embodiments presented herein are solely for the purposes of illustrating the principles of the invention.